This disclosure relates to a manufacturing method for preparing polycarbonates, and more particularly, to a manufacturing method for preparing polycarbonates by melt polycondensation of a dihydroxy compound and a diester carbonate in the presence of a catalyst.
Aromatic polycarbonates are used in a variety of applications due to their excellent mechanical and physical properties including, among others, impact and heat resistance, strength and transparency. There are three general processes known for the commercial manufacture of aromatic polycarbonates, which are illustrated in FIG. 1. The conventional interfacial process, as shown in FIG. 1A, and the phosgene-based melt process, as shown in FIG. 1B, start with the reaction of phosgene with carbon monoxide. The third general process, a xe2x80x9cno phosgenexe2x80x9d melt process as shown in FIG. 1C, was developed to eliminate the use of highly toxic phosgene in the process flow. Of these general methods, the xe2x80x9cno phosgenexe2x80x9d melt process shown is preferred since it prepares polycarbonates less expensively than the interfacial process and avoids the use of highly toxic phosgene.
Both types of melt processes (FIGS. 1B, and 1C) make use of a diarylcarbonate, such as diphenylcarbonate (DPC) as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A (BPA) in the presence of an alkaline catalyst to form a polycarbonate in accordance with the general reaction scheme shown in FIG. 2. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers. In many cases, however, the presence,of residual catalyst in the finished polycarbonate has a detrimental affect on the quality of the product, leading to poor color, a decrease in molecular weight, a decrease in transparency, or undesirable rheological properties. Residual catalyst may also interact with additives, detracting from their efficacy and rendering the polycarbonate prone to thermal decomposition. Thus, it is desirable to reduce the levels of residual catalyst to minimize these interactions. Such reduction is referred to as xe2x80x9cquenching.xe2x80x9d
Efforts to improve the stability of the finished polycarbonate has led to the use of alkali metal phosphorus-containing inorganic salts and/or alkaline earth metal phosphorus-containing inorganic salts as the catalyst. Of these phosphorus-containing salts, sodium dihydrogen phosphite and cesium dihydrogen phosphate are preferred for use in the melt polymerization process. In addition, the art now employs disodium magnesium ethylenediaminetetraacetic acid as a catalyst. These new catalyst materials require the addition of effective quenching compounds to prepare polycarbonates having acceptable residence stability.
A process for quenching a catalyst used in the manufacture of polycarbonate includes melt polycondensing an aromatic dihydroxy compound and a diester carbonate in the presence of a catalyst to produce a polycarbonate. The catalyst is selected from the group consisting of sodium dihydrogen phosphite, cesium dihydrogen phosphate and disodium magnesium ethylenediaminetetraacetic acid. A sulfonic acid ester quencher is added to the polycarbonate and the polycarbonate is extruded. The amount of sulfonic acid ester quencher added to the polycarbonate is effective to reduce the amount of branching species generated during extrusion of the polycarbonate to less than 100 parts per million.
These and other features will become better understood from the detailed description that is described in conjunction with the accompanying drawings.